Changing any part of the load from the book isn`t always wise. You usually can get away with it but altering a component will alter the load.
A-Square put out a manual that has more info on load preformance then any other I`ve read.
The book is called "Any Shot You Want"
They use rifle cartridges as examples, that is what they build, but the findings hold true in handgun cartridges also. They change buulet style, case capasity, and other obvious things and report on the effect they have on pressures. One thing they explore is the relation of the primer used to a given load.
The book uses a 7mm Remington Mag for a cartridge and 66 gr of H4831 with a 160 gr Sierra SBT. Useing primers from Winchester, Federal, CCI, and Remington they recorded a max pressure spread of 12,800 psi with a primer change only. The lots of components were all the same as was test conditions.
The fact you go from a mag primer from a std doesn`t help either. They found changing from a Remington 9 1/2M (magnum) primer to a Winchester WLR (std lg rifle) primer caused the average pressure to rise from 59,300psi for the mag primer to 64,400psi for the std. The Federal 215 magnum which I`ve heard many times called a "hot" rifle primer gave 100 psi less pressure then a CCI 250 mag primer.
BTW;
The Remington 9 1/2 M load showed 3,041 fps @ 59,300 psi.
The Winchester WLRM load ran 3045 fps @ 67,600 psi.
A 8,300 psi GREATER pressure for only 4 fps gain!!
The Win std primer loading ran 3,024 fps @ 64,400 psi.
That`s a DROP in velocity of 17 fps and a gain of 5,100 psi over the Remington mag primer load!
Don`t figure it is cut in stone your faster load is always giveing lower pressures, or that you can relate velocity to a change in pressure reliably. You may be very wrong........